Such a force sensor is known from EP 0 116 810 A1. The force sensor is constituted by an accelerometer comprising two piezoelectric transducers and a measuring object provided by a common seismic mass acting on both transducers and an additional seismic mass acting on only one transducer. The sensitivity of the two transducers and the inertial forces acting thereof are so selected that the two transducers, under the action of an acceleration force, each deliver equal output signals. Due to the matching of the sensitivity of the two transducers, mutually corresponding measurement signals can be generated and transmitted by a respective transmission channel provided for each transducer.
Redundant sensors are generally used in applications where a very high reliability is required for a particular measurement. A redundant measuring method consists in comparing the mutually corresponding measuring signals, in particular measuring signals of the same parameter at a given time, in order to increase the reliability of the measurement. In order to do the comparison, both signals should ideally be identical.
In the particular case of a two channel vibration measurement, however, practice has shown that it is very difficult to obtain two individual signals which are completely identical because two force sensors, in particular two accelerometers, cannot be placed exactly on the same spot in the engine. Out of this reason, the so called dual accelerometer according to EP 0 116 810 A1 was developed, where two separate transducer elements are built into one common housing. The redundancy of this approach includes a common preload system, a common seismic mass but two separate piezoelectric transducer elements, one for each channel. The remaining of the measuring chain, i.e. the transmission cables and the charge amplifiers, are kept as separate and fully segregated transmission channels.
For a well performing system, however, the two transmission channels of the dual accelerometer must match precisely. This matching includes performance characteristics such as sensitivity, frequency response, capacitance and resonance. The matching oftentimes makes the manufacturing difficult and it also requires the utilization of a high number of piezoelectric elements and time-consuming calibration of the transducer elements. It is also important that the temperature response and the individual long time aging go close together. Thus the production of such a dual accelerometer can be very costly. A mismatch or not close enough matching of the signals leads also very often to false alarms. In addition, the size and weight of such a device can present disadvantages in the mounting on aircraft engines, where the preference is small size and weight.